1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly to receiving transmissions within such wireless communication systems.
2. Description of Related Art
FIG. 1 is a schematic block diagram of a prior art radio receiver that includes a tuned preamplifier, a mixer, tuned local oscillator, 1st IF filter stage, another mixer, a local oscillation reference (LOs), a 2nd IF filter, a demodulator, a synchronization detect module, clock tracking module and automatic gain control (AGC) processor. The tuned preamplifier receives incoming RF signals via the antenna to produce an RF signal. The frequency spectrum of the RF signal is shown in FIG. 2 for multi-channel RF communications such as those prescribed within the IEEE 802.11 standard. As shown in FIG. 2, the RF signal may include a plurality of channels, in this example 4. The center of the RF signal is an intermediate frequency above the local oscillation (LO1) produced by the tuned local oscillator. Note that, in the alternative, the center of the RF signal is an intermediate frequency below the local oscillation (LO1).
Returning to the description of FIG. 1, the mixer, mixes the RF signal with the tuned local oscillation (LO1) to produce an intermediate frequency (IF) signal. The frequency spectrum of the IF signal is shown in FIG. 3. As shown in FIG. 3, the IF signal includes the 4 channels centered about the intermediate frequency (IF). In addition, an image of the IF signal is produced and is centered about the negative IF. In this example, the desired channel corresponds to channel 3 and the undesired channels are channels 1, 2 and 4. Returning to the description of FIG. 1, the 1st IF filter stage filters the IF signal to produce a filtered IF signal. The 1st IF filter is typically a SAW filter and has a frequency response as illustrated in FIG. 3. The frequency spectrum of the resulting filtered IF signal is shown in FIG. 4. As shown in FIG. 4, the desired channel 3 passes through the filter while channels 1 and 4 of the undesired channels are completely attenuated and a portion of channel 2 is attenuated. This also occurs on the image side of the filtered IF signal.
Returning to the discussion of FIG. 1, the next mixer, mixes the filtered IF signal with a 2nd local oscillation (LO2) to produce a baseband signal. The baseband signal is subsequently filtered by the 2nd IF filter stage to produce a filtered baseband signal. With reference to FIGS. 5 and 6, FIG. 5 illustrates the frequency spectrum of the baseband signal including the desired channel 3, the image of the undesired channel 2 overlapping and the image of the desired channel 3 overlapping with the undesired channel 2. When this is filtered, as shown in FIG. 6, the resulting filtered baseband signal includes the desired channel 3 and the image of the undesired channel 2.
When the desired channel 3 is actually a valid signal, the inclusion of the image of the undesired channel 2 presents minimal problems. If, however, there is no desired channel 3, but only the image of the undesired channel 2, the sync detect and corresponding clock tracking may indicate that a valid signal is present and activate the entire receiver to recapture the data from the filtered baseband signal. However, since this data corresponds to undesired information, the recovered data will be useless.
In general, the sync detect may be a correlation, which compares the incoming baseband signal to a stored representation of a valid preamble. If the beginning portion of the incoming baseband signal (e.g., the portion that would correlate to a preamble of a valid signal) matches the stored valid preamble, the correlator indicates that the signal is valid. If the correlator falsely identifies a valid signal, the subsequent processing by the receiver is wasted. For portable wireless communication devices, wasted receiver processing corresponds to wasted power, which reduces the battery life of a wireless communication device, and reduced data throughput. Such false identifications occur more frequently as the signal strength of the received RF signal decreases. As such, many wireless communication devices have a minimum signal strength requirement to reduce the number of false identifications, but do so at the cost of limiting the range of the wireless communication device and data throughput.
Therefore, a need exists for a method and apparatus to accurately detect the presence of a valid signal in view of undesired signals.